Cannot connect to VPN through L2TP

We have got a Server 2003 DC that was able to connect via VPN (PPTP and L2TP) and this crashed and had to be rebuilt completely. We now can't connect in through L2TP as this errors with
Error 789: The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer

I have tried to set this to a pre-shared key to see if this would help but still get the same error. There is no NAT on the server also.
When the server was rebuilt it was renamed to the original server name but the 'Full computer name' wasn't and so is the default name that Windows install chooses (sort of svctag-456432 etc). Could this be the issue in case do i need to rename the DC properly ?

OK, one more thing I missed. You renamed your DC, right? Probably there is also a CA role installed. So, certificate is not trusted on a VPN server because a CA has no valid certificate. You have 2 choices. Revert DC name to the proper one, restore also CA role with all certificates OR set up completely new certificates for VPN server and your clients. Redistribute them again.

Sorry for the late post. I've tried to export a new certificate but it still won't connect. Can you clarify exaclty what i need to do to get the certificate (i tried to create a cert template for IPSEC)

OK, you issued new computer certificate to your clients, right? Do you have Enterprise Edition of your 2003 server, where is CA component installed? It will simplify certificate's auto-enrollment in your domain environment (if not, you will do that manually).

I've had Microsoft on and they confirmed that all the certificates are correct. I've tried to connect to the server from itself through l2tp but it's showing that the remote coputer did not respond. I've checked the logs and this shows up when it try to connect:
The communication device attached to port VPN4-127 is not functioning.
I've set static DHCP along with Dynamic in case it's this but it still won't connect although pptp is fine.
I've tried using pre-shared keys as well.

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